A variety of analytical separation methods have been developed for differentiating between component molecules in a sample. Such methodology includes both gas-phase and liquid-phase chromatographic separations. Liquid-phase separation techniques include, for example, high performance liquid chromatography (HPLC), and capillary electrophoresis techniques such as capillary zone electrophoresis (CZE). Conventional gas-phase separation techniques include gas chromatography (GC) and ion mobility spectrometry (IMS). A comparison of typical resolution powers and efficiencies of these techniques indicates that gas-phase IMS can generally provide a greater efficiency relative to the alternative methods. However, although conventional IMS typically provides greater resolution than HPLC, the CZE and GC techniques can have a greater resolving power relative to IMS in the gas-phase.
In general an increase in resolving power for chromatographic methods is achieved by providing an increase in column length. For electrophoresis, an increase in resolving power can be achieved by providing an increase in applied voltage across the separation region. However, due to electrolytes utilized in the electrophoresis mobile phase, as the voltage is increased across a given channel length, the current increase heats the solution and can degrade resolving power. This heating effect can be a limiting factor on resolving power and miniaturization of capillary electrophoretic techniques.
Utilizing gas-phase IMS, which separates ions rather than neutral molecules, an increase in applied voltage across the drift field can be utilized to increase the resolving power in some instances. The general diffusion limited equation which relates ion mobility resolving power to operational parameters applies to both electrophoresis and IMS. Such equation is set forth below.
      R    2    =                    1                  16          ⁢                                          ⁢          ln          ⁢                                          ⁢          2          ⁢                                          ⁢          k                    ⁢              qV        T              ⇒          R      ∝                        qV          T                    
As demonstrated by the equation (where k is Boltzmann's constant), the IMS resolving power R is directly proportional to the square root of the charge on the ion q and the total voltage across the drift region V, and inversely proportional to the square root of the drift gas temperature T. In gas-phase IMS, higher voltages across the drift tube can lead to more rapidly eluting ions and the resolving power becomes limited with the pulse width of ions through the ion gate. Thus, in order to maintain resolving power, drift length is typically increased as the voltage is increased. Since it can be desirable to use small sensors for ion mobility separation, the effect is particularly notable in these applications.
The ability to use a decreased sensor size is typically dependent upon utilization of a relatively low voltage differential across the drift tube. As indicated, the decrease in voltage differential can sacrifice resolving power of the instrument. Attempts to scale down device size for gas phase IMS have typically resulted in decreased resolving power and decreased signal to noise ratios. Such factors can thereby limit the ability to produce a small size gas-phase IMS device having sufficient resolving power to accurately analyze many types of compounds and substances. Accordingly, it is desirable to develop alternative ion mobility technology.